Is There A Difference in Gunite And Shotcrete?
The term “gunite” has been used loosely to describe most concrete constructed swimming pools in recent times.
While it is sometimes applicable, there are several ways to build a concrete swimming pool that doesn’t necessarily fall into the category of true Gunite.
The term “gunite” is actually the trade name of the material that is produced by using a dry shotcrete method invented by Carl Akeley in the early 1900′s. Carl was granted patents for his “cement gun” and for the material produced by the process in 1911.
The term “Shotcrete” was coined by the American Railway Engineering Association in the 1930′s to describe the process used to produce “Gunite”. In 1951 The American Concrete Institute adopted the term “shotcrete” as well.
The term “shotcrete” is also now used to describe the “wet mix” process of applying cement that was developed during the 1950’s as an alternative to Gunite.
Today many people use the term “shotcrete” to describe the material that is produced by using the wet shotcrete method.
So What is Gunite?
Gunite is a mixture of dry sand and cement delivered through a hose by air pressure. Water is then added at the end of the hose via the nozzle to produce the desired consistency. The material is sprayed in place and shaped while still wet.
What is Shotcrete?
Shotcrete is actually the process used to place the wet mix or dry mix concrete material on the work area.
Oftentimes the term shotcrete is used to distinguish the difference in the material produced by a particular method. Gunite being the dry mix product and shotcrete being the wet mix product.
There are arguments on both sides as to which material is superior.
In the wet mix shotcrete process, concrete is premixed containing cement, sand, small gravel or other aggregates, and a specific amount of water. The material is then pumped through a hose and air pressure is added at the end of the hose through a nozzle to aid in the spraying of the material. Since the product is pre mixed, consistency can be maintained throughout the job. Likewise the addition of gravel may tend to strengthen the finished product.
With the dry mix shotcrete process the material produced is called gunite. A dry sand and cement mixture is forced through a hose using air pressure. Water is added to the mix at the end of the hose through the nozzle. The nozzle man is in total control as to how much water is added to the mixture. Since there is no way to determine the exact amount of water is being added, the nozzleman must rely on experience and training to control the mixture. One could argue that at best this is just educated guessing. If the mixture is too dry the finished product will crumble, too wet and cured compression strength is weakened.
Since the compression strength of cured concrete is directly affected by the amount of water added to the mix during placement one could ascertain that the wet process may produce a more consistent product.
Rebound or fallout is another undesirable byproduct more prominent in gunite or dry mix shotcrete than with the wet mix process. This is a byproduct of some of the dry mix not getting wet when passing through the nozzle and falling out of the spray before reaching the work area. Rebound must be removed from the job as it will not set and cure properly. Rebound should not be confused with the material that is removed or “cut from a pool wall during shaping. The material that is “cut” from the pool wall was thoroughly mixed and will cure properly.
Other methods of concrete pool construction may include forming and pouring the pool walls with concrete or laying the pool walls from cement block then filling the cavities in the block with concrete. Neither of which can be considered gunite.